Various techniques have heretofore been proposed for pipe inspection, including magnetic inspection by leakage field and/or eddy current techniques, gamma or X-ray techniques, ultrasonic techniques and television techniques. Each of such techniques may be used to advantage in certain applications but each has serious limitations as to others, such as, for example, the inspection of pipeline used for transport of natural gas from offshore stations. Natural gas cannot be treated offshore and in a raw state may include the combination of carbon dioxide and free water which produces carbonic acid and causes corrosion of steel, especially when the temperature is relatively high as is often the case. The corrosion is augmented by an erosion effect where the rate of flow is high and at bends and over weld beads or the like where the flow is turbulent. If localized damage to the pipe could be detected before it presents a serious problem, it would be possible to effect repair or to otherwise take corrective measures. For example, it is possible to move devices known as "pigs" through a pipe and by moving a series of such pigs through a pipe, it can be cleaned, etched, washed, dehydrated and then coated with a protective coating such as an epoxy. Such a procedure is, of course, relatively expensive especially in that the pipeline cannot be used for an extended period of time, and it is not desirable to utilize the procedure unless and until the necessity therefor can be established by a suitable inspection. Also, even after a protective coating is applied, there is the possibility of breaks in the coating and further periodic inspection is desirable to make certain that the coating is properly protecting the pipe. Accordingly, a reliable and accurate way of inspecting such pipeline for defects would be highly desirable.
As above indicated, prior art techniques are not satisfactory. Magnetic inspection if properly performed has many advantages including a high degree of sensitivity to flaws of types which may cause problems and the ability to cover large distances with suitable magnetic recording or other information storage equipment. However, there are disadvantages including the necessity of having operators with a high degree of skill in order to obtain an accurate interpretation of the information obtained. Ultrasonic techniques have a serious disadvantage in that it is difficult to couple the ultrasonic energy into the wall of a pipe and television techniques have a serious disadvantage, at least in the present state of development, in that a high sensitivity and high resolution is difficult to obtain especially if a substantial length of pipe is to be inspected and the information is to be recorded. If the information is to be transmitted through a cable, there is a problem as to loss of sensitivity at great distances and the difficulty in moving a cable having a large mass through a pipeline.